Recently it was determined that one of the facilitative glucose transporters, GLUT5, is an apically located protein in polarized epithelial cells which mediates the transport of fructose with high affinity. In contrast to the other members of the facilitative glucose transporter family, GLUT5 mediated fructose transport is much more efficient than glucose transport and fructose uptake is not inhibited by cytochalasin B. In the present application it is planned to exploit these unique properties of GLUT5 to determine functional domains of the facilitative glucose transporters. Specifically, chimeric glucose transporters will bc created between GLUT5 and the high affinity glucose transporter GLUT3 in order to localize the sequences which give rise to substrate specificities of GLUTS and GLUT3. These same chimeras will be used to determine the mechanism of cytochalasin B inhibition of glucose uptake. In separate experiments GLUT2 and GLUT5 will be transfected into the polarizable intestinal cell line, CaCo, and the route by which these transporters are targeted to either the basolateral or apical membranes respectively will bc determined. Subsequently, chimeric GLUT2/GLUT5 transporters will be expressed in CaCo cells to localize the sequences involved in directing the individual transporter to their unique subcellular site. These studies will increase the knowledge of the structure-function relationship among the glucose transporters. In addition, these experiments will help better understand the ways in which glucose and fructose enter cells and perhaps lead to ways in which the entry of glucose and other sugars into cell and out of the blood stream can be enhanced.